A method for assembling an optical unit according to the present invention comprises: an optical adjustment step of fitting a first fitting allowance and a second fitting allowance to each other and moving a first optical device holder and a second optical device holder relatively, thereby adjusting an optical path length between the first optical device and the second optical device; and, subsequent to the optical adjustment step, a welding step of irradiating from an area sandwiched between a holding surface perpendicular to the optical axis of the optical device and passing through a first holding part in the optical axis direction and a holding surface perpendicular to the optical axis and containing a second holding surface through to an interposed area between the first fitting allowance and the second fitting allowance, the interposed area located outside said area with laser light having substantially uniform accumulated energy per unit area in the irradiated area in a direction substantially perpendicular to the optical axis of the optical device, thereby welding and securing the first optical device holder and a second optical device holder to each other.